Recruitment and retention of study subjects is one of the most difficult components of research. This difficulty is compounded when dealing with vulnerable and hard-to-reach populations such as inner-city, minority adolescents. Recruitment and retention of study subjects can be both labor- and time-intensive and if not done properly can impede researchers' ability to answer critical scientific questions. The overall objective of the Cohort Core is to provide the logistical support to recruit, consent, screen, enroll and retain study subjects and to collect the appropriate interview and biological data as needed for all projects of the San Francisco Adolescents and STDs CRC (SFCRC). It is the goal of this Core to provide all necessary support to ensure that the individual projects of the SFCRC receive the data needed to achieve their specific aims. Because individuals will be identified and recruited in an organized manner, multiple studies can be ongoing without creating confusion within the community. A centralized recruitment model allows for the implementation of several studies simultaneously as well as for future expansion of projects without jeopardizing the access, aware and acceptability established to conduct the initial projects. Specifically, the aims of this Cohort Core are to: 1. Recruit, consent, screen, and enroll a random household probability sample of adolescent index subjects from the target population identified through random-digit dialing 9Projects 1, 2, 3) 2. Recruit, consent, and enroll two generations of adolescent index subjects' sex partners (Projects 1, 3) 3. Recruit, consent, screen, and enroll female social partners of sexually inexperienced female adolescent index subjects (Projects 2, 3). 4. Track and retain study subjects for follow-up (Projects 1, 2, 3) 5. Ensure successful collection of interview data and biological specimens from study subjects (Projects 1, 2, 3)